Method for pipelaying from a coil to the sea bed, controlling thermal expansion

ABSTRACT

A method for laying a pipeline on the sea bed from a lay barge in order to achieve controlled thermal expansion, comprises the following steps: Feeding out of the pipeline having a radius of initial residual curvature from a pipeline reel, preferably via a stinger to a curvature device where a radius of reversal curvature is applied to the opposite side of the pipeline with respect to the radius of initial residual curvature when the pipelines is passing through the curvature device. The curvature device straightens out the pipeline to longer, mainly straight portions with length having a radius of residual curvature which is larger than a desire minimum radius of residual curvature. The curvature device exposes shorter portions of the pipeline with a length for a radius of counter curvature forming the thermal expansion loops having a radius of residual curvature which is less than a desired maximum radius of residual curvature for the thermal expansion loops. The result is controlled pipe deflection under thermal expansion of the pipeline in operation position on the seabed and under operational loads, in that the deflection of the pipeline may appear in the thermal expansion loops of the pipeline.

This application is a section 371 national stage application from PCTInternational Application No. PCT/NO02/00023, filed Jan. 17, 2002.

The invention relates to a method for laying a pipeline on the sea bedby means of a pipe laying vessel, for controlled thermal expansion. Moreprecisely the application relates to a method for varied reverseddeflection of the pipeline subsequent to the pipeline being plasticallybent when fed along a curved portion of a stinger, or subsequent tostorage on a reel and subsequently reeled out.

PRIOR ART ON THE FIELD OF TECHNIQUE

A method for laying a pipeline is shown in U.S. Pat. No. 4,992,001;“Method of deepwater pipelay”. The method comprises laying in waterdepths greater than 100 feet, where the pipeline is assembled on board alay barge. During deployment as the pipelines is fed out, it is bentbeyond typical elastic bending limitation while passing along a stingerwhere it is subsequently reverse bent to remove any permanent curvature.Upon further deployment into the sea, a generally horizontal axial loadis maintained in the pipeline as it eventually comes to rest on the seafloor such that the final in-situ residual bending curvature of thepipeline is nearly zero or falls within an acceptable range.

U.S. Pat. No. 5,975,802 describes a pipelaying vessel with production ofthe pipeline on the deck and a production line guiding the assembledpipeline to a stinger having means for regulating its angle ofinclination when the pipeline is fed into the sea. The stinger comprisesa clamping mechanism for the pipeline and a mechanism for straighteningthe pipeline in order to remove non-elastic bending as in U.S. Pat. No.4,992,001. The pipeline may be fed out in forward direction from thefabrication station on the deck and then have its direction and movementreversed to lay over the stern or be fabricated in an aft direction,laid over the stern. A paper by Lana, G. A: “Mobile Bay Fairway FieldFlowline Project”, OTC 7014, Offshore Technology Conference, Houston,Tex., 1992, also disclosed in Lanan's U.S. Pat. No. 5,403,121, describesa pipeline laid in zigzag configuration along the seabed in order toallow the pipeline to expand or contract sidewise when exposed toincreasing or decreasing temperatures. Prefabricated double jointedpipes having a loop length of approximately 23 meter and a bend angle of8° are joined to a pipeline on deck and fed out into the sea via astinger. Lanan describes in the patent specification column 1, line22-30: “The suspended pipe span between the vessel stern and theseafloor is typically supported by a stinger attached to the vesselstern and axial tension applied to the pipe. Applying this tension tothe pipeline incorporating expansion lops will exceed the elastic stresslimit of a typical expansion loop. Typical expansion lops would also notpass easily through the lay barge pipe tensioning machine or stinger”.Lanan shows a technical prejudice in that tensional force during layingof a pipeline with expansion loops will exceed the elastic tensionlimits for a typical expansion loop. This is not the case according tothe present invention.

GB 2,287.297 describes a method for laying undersea pipelines comprisingforming the pipeline from straight pipe sections and then importingalternately oppositely directed loops on the pipeline which is then laidundersea. A bending station with a set of three transverse, horizontallydirected pairs of hydraulic pistons, each pair of pistons being oppositeeach other and designed to press towards the pipe, bend the pipesidewise to required degree.

DESCRIPTION OF THE PROBLEM

Usually the pipeline on the seabed transporting oil and gas under highpressure and temperatures will be exposed to axial compressive forces,which may cause the pipeline to buckle, so called pipe buckling, andwhere the strain and stress in the buckled pipe exceeds the criteria foracceptance and my cause collapse of the pipeline. It is therefore commonpractice either to place the pipeline in a ditch and/or cover it bymeans of gravel in order to keep it in place. Such ditching orgravelling is expensive and may cause conflict with other interestedparties, for example it may cause impact on, or prevent fishery.Ditching and gravelling may in most cases be superfluous if one succeedsin transfer or in any manner reduce the axial loads in a controlledmanner, for example through controlled deflection several places,whereby local buckling and excessive stresses and strains, whichnormally would appear in straight pipelines, may be avoided.

A pipeline which is exposed to increase in temperature will have atendency to extend in longitudinal direction. Because of interactionbetween the pipe and the soil in the form of friction and cohesion theextension in longitudinal direction will be prevented and axialcompressive force will occur. In pipelines which are so long thatinteraction between the soil and the pipeline prevents expansion of themiddle portion towards the ends do not provide other possibilities oflongitudinal extension than by buckling of the pipeline and deflectionsin the horizontal or vertical plane. Such buckling may usually beprevented by complete ditching with a cover, and increase in temperaturewill produce an increased axial compressive strain. A partly buried oropenly laid pipeline will in case of an increase in temperature beexposed to buckling, called upheaval buckling, which locally may besubstantial, in particular if the entire increase in length along aportion of the pipeline is concentrated in one single buckling. In thiscontext this phenomenon is called localising. The bending strains maylocally in the deflection area be so high that they exceed allowedstrain limits.

An offshore pipeline which is intended to transport gas and/or fluidhaving a high temperature, such as for example unprocessed well streambetween a production plant and a processing plant, is constructed toprevent upheaval buckling, for example by being trenched and covered bycontinuous or discontinuous backfill of gravel, by laying the pipelinealong a snaked route, by laying the pipeline in a larger casing, or byincluding expansion loops in the pipeline along its length. Thesesolutions may be extremely expensive, or they may also leave uncertaintyfrom a functional point of view.

Conventional installation methods provide a built-in tensional force inthe pipe subsequent to laying on the seabed. When calculating thecompressive force, tending to produce buckling, the axial tensionalforce from the laying operation is subtracted from the compressive forcemobilized as a result of increase in temperature. It may therefore beassumed that laying with a high axial tension may contribute toreduction of or the prevention of buckling. The relative increase inlength of steel as a consequence of normal temperatures during transferof well fluid stream (which may be in the order of 100° C.) may,however, be substantially larger than the extension normally appearingdue to tensional force in the pipeline caused by laying. Axial tensionmay thus normally only compensate for a very limited increase oftemperature.

One of the objects according to the invention is, at certain intervals,to give portions of the pipeline less stiffness so that extension inaxial direction due to temperature may occur in a distributed andcontrolled manner and cause pipe deflections without producing largecompressive forces. This is achieved by forming loops, hereinaftercalled thermal expansion loops, in the pipeline during installation, insuch manner that the pipeline in unloaded condition forms evenlydistributed and sidewise oriented expansion loops in the longitudinaldirection of the pipeline. When the pipeline is exposed to axial tensioncorresponding to normal tension during laying, the pipe will, inaddition to extension as a direct consequence of the tensional force, beextended as the thermal expansion loops are partly straightened out,i.e. it becomes pre-stressed. At the same time bending moments willappear in the pipe having a maximum value corresponding to the tensionalforce times the reduction in the amplitude of the thermal expansionloops in loaded conditions. Said bending moments will when the tensionis relieved, force the geometry back to the original configuration ofthe expansion loops when in unloaded condition.

Large increases in temperature will lead to a net compressive force inthe pipeline, which for maximum temperature, however, will besubstantially lower than in case the pipeline had been without loops,see FIG. 6 and the description of the Figure below. Due to the evenlydistributed loops the tendency of an aggregated increase in length atone single location, localizing the extension in length will be avoided.Strains are reduced correspondingly since the extension in length willbe evenly distributed between the various expansion loops.

The geometry of the pre-bent curves of the thermal expansion loopsshould be such that the stiffness in longitudinal direction will besubstantially less than the axial stiffness for a straight pipe. It isan advantage that the radius of the residual curvature of the thermalexpansion loops are greater than 250 times the diameter of the pipe,since this corresponds to the radius of curvature when a substantialstraight pipe with typical material properties are bent to its elasticlimit, i.e. such that no plastic deformation is obtained.

SHORT SUMMARY OF THE INVENTION

The above mentioned problems may be solved by a method for laying apipeline on the seabed from a lay barge, for controlled thermalexpansion, comprising the following steps:

Feeding a pipeline having a radius of initial residual curvature, eitherfrom a reel or caused by bending over the top of a stinger, to a bendingapparatus where a radius of reversal curvature formed in any knownmanner is applied as the pipeline is passing through the curvature meansto the opposite side of the pipeline with respect to the radius ofinitial residual curvature.

The bending apparatus straightens out the pipeline to longer, mainlystraight portions having a radius of residual curvature larger than adesired minimum radius of residual curvature.

The bending apparatus exposes shorter portions of the pipeline to aradius of reversal curvature forming thermal expansion loops having aradius of residual curvature which is less than a desired maximum radiusof residual curvature for the thermal expansion loops.

The result of the steps of this method is that controlled pipe bendingunder thermal expansion of the pipeline in operational position on thesea bed and under operational loads is obtained in that the deflectionof the pipeline may appear in the thermal expansion loops already atsmall compressive forces.

According to a preferred embodiment of the invention, where theexpansion loops are given a radius of residual curvature which is largerthan the elastic radius of curvature of the pipe, whereby an axialstrain only will straighten out the expansion loops elastically, and inorder to avoid straightening out the expansion loops plastically, thetechnical prejudice in Lanans patent is proved to be incorrect. Theelastic strain limits for a typical expansion loop will not be exceeded,and the expansion loops will be able to pass through the strain machinesor the stinger on the lay barge, since they have a radius of residualcurvature which is larger than the elastic radius of curvature of thepipe. Hence, the expansion loops have a smaller residual curvature thanthe elastic curvature of the pipe, and will thus not be straightened outat elastic tension.

According to a beneficial embodiment of the invention the pipeline isfed out from a pipeline reel. Hence, a rapid and continuous feeding outof the pipeline is achieved.

According to a preferred embodiment of the invention all expansion loopsare formed towards the same side of the pipeline, enabling bending ofthe pipeline to take place in the same direction in the thermalexpansion loops of the pipeline. According to a preferred embodiment thethermal expansion loops are formed in such way that they become convexupwardly in a vertical plane when the pipeline is fed from of thecurvature means and in a most preferred embodiment with such radius ofresidual curvature, length and frequency that each and every expansionloop rotates from a vertical orientation to a horizontal orientation asthe laid pipeline is stabilised on the seabed.

BRIEF DESCRIPTION OF THE DRAWINGS

Below a figure explanation of the enclosed drawings is given. Thedrawings shall not be construed in any way to limiting the invention,but shall only be construed as illustrations and to simplify theunderstanding of the invention, which only shall be construed in view ofthe enclosed claims.

FIG. 1 illustrates a preferred embodiment of the method according to theinvention, showing a side view of a vessel provided with a pipeline reeland a straightening means for the pipeline.

FIG. 2 illustrates a side view of the lay operation of a pipeline,indicating sections of expansion loops on the pipeline.

FIG. 3 illustrates a horizontal view, seen straight down towards the seabed of a deployed pipeline according to the invention, where theportions having expansion loops show deflection.

FIG. 4 a is a horizontal view and illustrates an enlarged portion of thepipeline on FIG. 3. The Figure illustrates a cold laid expansion loop onthe pipeline and deflection of the expansion loop on the pipeline atworking loads caused by temperature and pressure in the pipeline.

FIG. 4 b is partly a section and partly a vertical view seen along thepipeline, and illustrating an expansion loop in cold and in warm,deflected condition.

FIG. 5 illustrates the total work required for bending and torsion ofthe pipeline in the free span from the lay barge towards the sea bed fordifferent lengths with residual deflection in the pipeline.

FIG. 6 shows two curves where the upper curve shows the potentialcompressive force in a pipeline without appreciable rest curvature andwithout expansion loops, while the lower curve illustrates the effectiveaxial compressive force caused by expansion in a pipeline according tothe invention with expansion loops having lengths l_(E) between straightportions having lengths l_(L).

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE METHOD ACCORDING TO THEINVENTION

The invention relates to a method for laying a pipeline (1) on the seabed from a pipeline lay barge (10).

An example of the dimensions of an actual pipeline (1) may be asfollows:

Steel material quality: X65 Internal diameter: 250 mm Wall thickness:13.9 mm; D/t = 20 Operation data: Water depth: 200 m Internal pressure:30 Mpa Increase of temperature: 100° C. Submerged weight (air filledpipe): 200 N/m Submerged weight (in operation): 500 N/m Lateralcoefficient of friction μ_(s): 1.0 Axial coefficient of friction μ_(a):0.5 Installation data: Reel Radius: 8 m Angle of departure for thepipeline: 60°, counted between the pipeline axis and the horizontalplane/the plane of the deck of the lay barge Local residual strain:0.12%, corresponding to a residual curvature 8.64 E⁻³m⁻¹ = residualradius of curvature 116 m in the portion where the tube is onlypartially straighten out, see below for further explanation.

The object of the invention is to achieve a controlled thermalexpansion, limited to certain expansion loops (E) which according to themethod is formed along the pipeline. An illustration of expansion loopsis shown on FIG. 3. The method comprises the following steps:

The pipeline (1) is fed out from a pipeline reel (2) onboard the laybarge, preferably via a stinger (3), to a curvature means (4). Thissituation is described in the prior art, and is illustrated on FIG. 1.The pipeline will have a start residual curvature radius (R_(rInit))since it is reeled out from a pipeline reel (2) having commonly a reelradius of curvature which is less than the elastic radius of curvatureand as a consequence, it has been bent plastically. The initial residualradius of curvature (R_(rInit)) may also appear when the pipeline ispassing over the portion at the top of the stinger (3), where the radiusof the curved portion may correspond to the radius of the pipeline reel.Situations were the stinger is superfluous, may also be feasible, thepipeline running directly from the reel towards the deflection means.The pipeline may be continuous fed to the deflection means (4) where aradius of reversed curvature (R_(mk)) is added to the opposite side ofthe pipeline (1) with respect to the radius of initial residualcurvature (R_(rInit)), the pipeline (1) running through the deflectionmeans (4).

The deflection means (4) straightens out the pipeline (1), producinglong, mainly straight portions (L) having a designed length (l_(L)), andforming a radius of residual curvature (R_(r)) which is larger than anintended minimum radius of residual curvature (R_(rMin)). Said radius ofminimum residual curvature will for straight portions in theoryapproaches indefinite, but not completely, in reality.

One feature of the invention is that the deflection means (4) exposesshorter portions of the pipeline (1) having a length (l_(E)) for aradius of residual curvature (R_(E)), forming thermal expansion loops(E) with a radius of residual curvature (R_(E)) which is less than anintended largest radius of residual curvature (R_(EMax)) for the thermalexpansion loops (E). Said expansion loops are illustrated in FIG. 2 andalso in FIGS. 3 and 4. The pipeline may in general consist of longerstraight portions (L) having expansion loops (E) mainly evenlydistributed along the pipeline, whereby controlled pipe bending duringthermal expansion if the pipeline (1) in operational position on theseabed and during operational loads is obtained by allowing bending tooccur already at small axial compressive loads in the thermal expansionloops (E), and in general not in the straight portions. A pipeline laidaccording to the invention is somewhat shorter. The pipeline is fedcontinuously out, and consequently faster and cheaper than a zigzagpipeline as shown in GB 2,287,297, where feeding of the pipelinenormally have to stop during bending of the pipe by means of thelaterally arranged pistons. Other advantages according to the inventionappear more clearly when read in conjunction with the disclosure of theinvention below.

According to a preferred embodiment of the invention, where theexpansion loops (E) is given a residual radius of curvature (R_(E)),which is larger than the elastic radius of curvature of the pipe, it isachieved that for axial tension the pipe will only straighten out theexpansion loops (E) elastically, thereby avoiding that plasticallystraightening of the expansion loops (E) will occur when the pipeline isexposed to tension. The pipeline is exposed to tension both due to theweight of the portion which at any time is between the vessel and theseabed, and due to horizontal tension produced by the lay barge. If theaxial tension is given the possibility of producing plasticstraightening of the pipe, this plastic straightening will occur in weakplaces on the tube, and consequently the properties of the pipe at suchportions will remain unidentified. This should be avoided. For axialtension of the expansion loops according to this preferred embodiment ofthe invention, an equal straightening moment will be employed on theentire pipeline, thus the problem of unknown pipeline properties due toplastic deformations will not appear.

Analysis of Installation

A pipeline leaving the lay barge, see FIGS. 1 and 2, where the pipelinehas a residual curvature which is upwardly convex will be sensitive forrotation in the free span between the lay barge and the sea bed, wherethe curvature is opposite of the residual curvature and upwardlyconcave. Calculations have been made, based on pure energyconsiderations of the bending and torsion, predicting the rotation ofthe pipeline in the free span. The analysis of installation shows thatwith an exit angle of 60° and a water depth of 200 metres the spanbetween the sea surface and the seabed will be approximately 360 metres.Maximum pulling stretch in the under bend will be well inside acceptedvalues for relevant border conditions.

Rotation of the pipeline due to residual curvature will increase withthe distance from the lay barge as the pipeline approaches the seabed.Since rotation is caused by the residual curvature in the pipeline theinstalled pipe will not be left with torsional moment on the seabed;only a bending moment, provided the rotation takes place to the sameside as the pipelying is proceeding. This is predominantly probable;once the rotation first has “chosen” side with an expansion loop (E) thesubsequent straight portion (L) of the pipeline and the next expansionloop (E) gain an increased probability for bending over to the sameside. The torsional moment in the pipeline has thus its maximum valuewhen leaving the lay barge and decreased towards the seabed where thetorsional moment becomes zero.

FIG. 5 shows the total work performed in order to bend and twist thepipeline from the sea level down towards the seabed for angles ofrotation between 0° and 180°. Curves are calculated for differentlengths l_(E) of the thermal expansion loops (E).

-   a) l_(E)=the entire length of the pipeline-   b) l_(E)=300 m-   c) l_(E)=200 m-   d) l_(E)=150 m-   e) l_(E)=100 m-   f) l_(E)=50 m-   g) l_(E)=0 m, i.e. no residual curvature

FIG. 5 illustrates this work for cases a) to g). The curves indicatemuch of the probability for rotation due to residual curvature.

If the curve is evenly increasing energy must be supplied in order toachieve rotation.

If the curve is relatively flat at small angles and thereafterincreases, then the system is relatively unstable and the pipeline maytwist to an angle and will require additional energy in order to twistto a larger angle.

If the curve initially has a negative inclination the twisting mayeasily be initiated. If the curve also has a distinct minimum point, theangle of twist of the pipeline at the seabed will be in the angle areawhere the curve has its point of minimum. FIG. 5 shows that in order toachieve 80° to 100° rotation of the pipeline the expansion loops (E)should have lengths l_(E) in the range between 100 m and 150 m for thisexample. The twisting is favourable, since the expansion loops formed inthe vertical plane thus turn over to the side and ends in the horizontalplan along the seabed. This is ideal for further sidewise deflection atthe changes in pressure and differences in temperature appearing duringoperational loads.

FIG. 6 shows two curves where the upper curve shows the potentialeffective pressure force P_(pot) in a pipeline restrained from axial andsidewise expansion, while the lower curve illustrates how the effectiveaxial compressive force is strongly reduced due to distributed expansionin a pipeline according to the invention with expansion loops havinglength l_(E) at axial compressive force P_(E) between straight portionsof length L_(L) with axial compressive force P_(L).

According to a preferred embodiment of the invention, residual curvaturein the expansion loop (L) will be formed on the same side of thepipeline (1) enabling bending of the pipeline to appear in samedirection in the thermal expansion loops (E) of the pipeline. In themost preferred embodiment of the invention, the residual curvature isformed so that the thermal expansion loops (E) are convex upwardly in avertical plane when the pipeline is leaving the curvature means (4).

In a method according to the invention the curvature means (4) used,comprises two reversal rollers (6) arranged on one side of the pipeline(1), on the disclosed embodiment, on the underside. An intermediate,opposite acting curvature reversal roller (7) runs on the opposite sideof the pipeline (1). The rollers (6, 7) form the radius of countercurvature (R_(mk)) on the pipeline (1), while running through thecurvature means (4), and the pipeline will have a radius of residualcurvature subsequent to elastic reversal bending if not subjected tostrain or tension along the stinger (3) subsequent to passage throughthe curvature means.

According to the preferred embodiment of the invention an adjustablepower means (8) designed to vary the position of the curvature reversalrollers (7) and thus the pressure force against the pipeline (1) andindirectly against the reversal curvature (6) in order to regulate theradius of the counter curvature (R_(mk)), is applied. Such regulation ofthe position of the reversal curvature rollers is described in prior artsolutions, for example in U.S. Pat. No. 4,992,001, in order to adjustthe distance of the rollers with respect to the pipeline and in order toprovide the required counter curvature bending of the pipeline.According to the present invention, however, said rollers are used toregulate the radius of counter curvature to form straight portions andexpansion loops, respectively.

According to a preferred method of the invention, portions havingdesires length (l_(E)) of the pipeline (1) are given a smaller reversalbending with a larger radius of counter curvature (R_(mkE)). Hence, theradius of counter curvature (R_(rInit)) inherit from the reel (2) or thestinger may be exploited, thereby forming thermal expansion loops (E)having a radius of counter curvature (R_(E)) in the same direction asthe radius of initial start-residual curvature (R_(rInit)). According tothis preferred embodiment of the invention the pipeline is upwardsconcave when entering the curvature means (4) and the counter curvatureroller (7) is released to desired degree in order to form each thermalexpansion loop. In such manner an insignificant amount of energy is inpraxis saved by employing the preferred method.

It is possible to expose the pipeline (1) for a continuous varyingradius of counter curvature (R_(mk)) in the curvature means at thetransition between formation of the straight portions (L) and theexpansion loops (E).

The pipeline (1) in the free span between the lay barge (10) and theseabed is exposed for a certain elastic elongation which in the mainstraightens out the expansions loops (E). The installed pipeline willhave a certain inherent axial tensional force when in cold conditions.,whereby the elastic elongation will not be completely reversed when thetensional forces in the pipeline is reduced subsequent to the pipelineresting on the seabed. The expansion loops (E) will prior to settling onthe seabed contain a minimum of energy which is a function of an axis ofrotation about a longitudinal axis (A), sufficient to rotate theexpansion loops (E) from the vertical plane and mainly into thehorizontal plane towards the seabed. Dependent upon the properties ofthe pipeline material, dimensions, tensional forces appearing, thelaying geometry and the radius of residual curvature, together with thelengths of the expansion loops, said minimum of energy may appear atdifferent angles, see FIG. 5. It is an advantage if said minimum ofenergy will turn the concave residual curvature along the seabed,laterally with respect to the route of the pipeline.

In the preferred embodiment of the invention the length (l_(E)) of theportions of expansion loops (E) is made shorter than the length (l_(L))of the straight portions (L). In a more preferred embodiment thisproportion is less than 1:4, and in a further preferred embodiment theproportion is less than 1:10. It is preferred that the expansion loopsare repeated every 100 m. More preferably said distances may exceed 500m, and for a still more preferred embodiment the expansion loops arerecurring with a distance more than 1000 m.

Usually a pipeline reel standing in an upright position, having ahorizontal axis (20) of rotation, arranged in transverse direction withrespect to the direction of motion of lay barge (10) is used. It is alsopossible to use a reel having a vertical axis (20) of rotation. In theillustrated method according to the invention the pipeline (1) is fedout in rear direction from the top of the reel.

According to a possible embodiment of the invention at least a middleportion of the straight portions (L) of the pipeline (1) is covered sothat the portions on each side of the middle part is allowed to expandaxially towards each expansion loops (E). Alternative ways of securelyposition these middle portions on the seabed may also be used.

The present method for controlling the back bending in the verticalplane provides for possibilities of adapting the shape of the pipelineto the seabed topography in order to prevent buckling over ridges andlong free spans over ditches. According to a preferred method of theinvention the curvature means (4) may expose portions of the pipeline(1) having lengths (l_(T)) for a radius (R_(mkT)) of counter curvatureforming topographically adapted loops (T) having a radius of residualcurvature (R_(T)) corresponding to topographical ridges and bottompoints along the route of the pipeline (1), whereby the pipeline (1) inoperational position on an uneven seabed achieves reduced peak momentsand reduced lengths of span when crossing topographical ridges or bottomfurrows.

1. A method for laying a pipeline on a sea bed from a lay barge in orderto provide controlled thermal expansions, comprising: feeding thepipeline from a pipeline reel, preferably via a stinger, where thepipeline has a radius of initial residual curvature, to a curvaturemeans where a radius of counter curvature is applied on opposite side ofthe pipeline with respect to the radius of initial residual curvaturewhen the pipeline is running through the curvature means, wherein thecurvature means straightens out the pipeline to longer, mainly straightportions having a length with a radius of residual curvature greaterthan a predetermined smallest radius of residual curvature; and thecurvature means exposes shorter portions of length of the pipeline for aradius of counter curvature forming thermal expansion loops having aradius of residual curvature which is less than a predetermined largestradius of residual curvature for the thermal expansion loops; whereby acontrolled deflection of the pipe during thermal expansion of thepipeline in operational position on the sea bed and when exposed tooperational loads is achieved in that deflection of the pipeline mayappear in the thermal expansion loops of the pipeline, wherein allthermal expansion loops are formed toward the same side of the pipeline,whereby bending of the pipeline may occur in same direction in theexpansion loops of the pipeline.
 2. The method according to claim 1,wherein the expansion loops are given a radius of residual curvaturewhich is larger than the radius of elastic curvature of the pipe,whereby only the expansion loops will be straightened out elasticallywhen exposed to axial tension, avoiding plastic straightening of theexpansion loops.
 3. The method according to claim 1, wherein the thermalexpansion loops are formed in such way that they become convex upwardsin a vertical plane when the pipeline leaves the curvature means.
 4. Themethod according to claim 1, wherein the shorter portions of length ofthe pipeline are given a smaller deflection with a larger radius ofcounter curvature and thus form thermal expansion loops having a radiusof residual curvature in same direction as the original radius ofinitial residual curvature.
 5. The method according to claim 4, whereinthe expansion loops just before and during laying towards the sea bedwill have an energy minimum as a function of a rotational angle around alongitudinal axis which is sufficient to rotate the expansion loopsmainly sidewise, towards the sea bed.
 6. The method according to claim1, wherein the length of the portions of the expansion loops is madeshorter than the length of the straight portions.
 7. The methodaccording to claim 6, wherein the proportion between the length of theshorter portions of the pipeline and the length of the straight portionsis less than 1:4.
 8. The method according to claim 6, wherein theproportion between the length of the shorter portions of the pipelineand the length of the straight portions is less than 1:10.
 9. The methodaccording to claim 6, wherein the expansion loops are recurring with adistance exceeding 100 m between each start of a new expansion loop. 10.The method according to claim 6, wherein the expansion loops arerecurring with a distance exceeding 500 m between each start of a newexpansion loop.
 11. The method according to claim 6, wherein theexpansion loops are recurring with a distance exceeding 1000 m betweenthe start of a next expansion loop.
 12. The method according to claim 1,wherein the curvature means exposes portions of the pipeline with lengthfor a radius of counter curvature forming loops adapted to thetopography and having a radius of residual curvature corresponding tothe topographical shoulders and bottom points along the projected routefor the pipeline; so that the pipeline in operational position on anuneven seabed and subjected to operational loads achieves reduced momentmaximums and reduced length of span when crossing topographicalshoulders or bottom points.
 13. The method according to claim 1, whereinthe pipeline is positioned on the sea bed when in the operationalposition.